Optical device for measuring the roll angle of a projectile

ABSTRACT

This optical device comprises, at the rear of the projectile (3), a catadioptric system (15) equipped with a polariser (16) and, at the firing station of the projectile (3), a light source (5) whose beam illuminates the rear of the projectile (3) and a light-flux analyzer (6) calculating from the polarization direction of the light flux reflected by the projectile (3) the roll angle of the latter. It is noteworthy in that it comprises, in order to remove the ambiguity of π from the measurement of the roll angle, a reflector dihedron (20) which is positioned along the side of the projectile and which reflects the light beam from the source (5) back to the light-flux analyzer (6) once per turn of the roll of the projectile (3) when the light source (5) is not blocked from it by the body of the projectile (3).

FIELD OF THE INVENTION

The present invention relates to the measurement of the roll angle of aprojectile.

BACKGROUND OF THE INVENTION

The general evolution in threat and weapon systems raises the need forimproving the performance of weapons by virtue of a guiding systemwhilst seeking a minimum cost.

It is particularly advantageous, then, to reduce as far as possible theamount of sophisticated equipment loaded on board an expendable guidedprojectile (computer, homing head, inertial unit, proximity fuse etc.)by transferring the maximum number of functions to the firing stationalone.

The guidance of a guided weapon launched by a conventional orelectromagnetic gun and set into a rotation movement about its axis, maybe performed with the aid of gas-generating lateral impellers, theoperation of which is actuated when they are oriented in the desireddirection. This requires knowing at any instant the roll angle of theprojectile.

This function of angular measurement of the roll is generally ensured byan inertial unit (rate-gyro) loaded on board the projectile, whichinertial unit is expensive and expendable. Furthermore, this inertialunit becomes difficult to design and to construct in the case of aprojectile launched by an electromagnetic gun where the acceleration atthe start may reach 200,000 g.

It is known how to measure the roll angle of a projectile with the aidof a catadioptric system equipped with a polarizer disposed at the rearof the projectile, with a light source illuminating the rear of theprojectile and a light analyzer calculating the roll angle of theprojectile from the polarisation direction of the light flux reflectedby the rear of this projectile. However, this measurement has thedrawback of exhibiting an ambiguity of π.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to enable ambiguity in an opticalmeasurement of the roll angle of a projectile to be removed with the aidof components loaded on board the projectile which are robust, reliableand inexpensive.

The subject of this invention is an optical device for measuring theroll angle of a projectile launched by launching means located in afiring station. This device comprises, at the rear of the projectile, acatadioptric system equipped with a polarizer and, at the firingstation, a light source which is laterally offset in relation to thefiring axis of the projectile and whose beam illuminates the rear of theprojectile, and a light-flux analyzer calculating, from the polarizationdirection of the light flux reflected by the rear of the projectile, theroll angle of the latter. It is noteworthy in that it furthermorecomprises a reflector dihedron which is positioned along the side of theprojectile and turned towards the rear of the latter with its edgenormal to the rotation axis of the roll of the projectile and whichreflects the light beam from the source back to the light-flux analyzeronce per turn of the roll of the projectile when the light source is notocculted from it by the body of the projectile.

The catadioptric system may be a reflector trihedron.

The light-flux analyzer comprises a first optical receiver which issensitive to the intensity of the light flux and which receives thelight flux reflected by the projectile by the agency of a polarizerconstituting a polarization analyzer, a second optical receiver which isalso sensitive to the intensity of the light flux and which is directlyexposed to the light flux reflected by the projectile, aturbulence-compensating circuit splitting the signal supplied by thefirst optical receiver from the signal supplied by the second opticalreceiver and a circuit for estimating the roll angle of the projectileemploying the signal supplied by the turbulence-compensating circuit.

Other characteristics and advantages of the invention will emerge fromthe description of embodiments given by way of example. This descriptionwill be made hereinbelow, with respect to the drawing in which:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an overall diagram of a guided-projectile firing stationequipped with an optical device according to the invention;

FIG. 2 represents, diagrammatically, a guided projectile with itstrajectory-changing means;

FIG. 3 represents, diagrammatically, the main elements of the opticaldevice according to the invention;

FIG. 4 is a graph illustrating the light-flux intensity variations whichare detected in the optical device according to the invention as afunction of the roll angle of the projectile;

FIG. 5 represents, diagrammatically, the main elements of an opticaldevice according to the invention, provided with turbulence-compensatingmeans;

FIGS. 6, 7, 8 are graphs illustrating the operation of the opticaldevice represented in FIG. 5 and

FIG. 9 represents, diagrammatically, a complementary optical deviceenabling an ambiguity of π in the measurement of the roll angle of theprojectile to be removed.

In the drawing, the identical elements from one figure to another areindexed by the same references.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a firing station 1 equipped with a gun 2 having justlaunched a guided projectile 3 in the direction of a target 4. Theoptical device for measuring roll angle of the guided projectile 3comprises, at the firing station 1, a light source 5 consisting of alaser pointed at the guided projectile 3 and a light-flux analyzer 6which is mechanically coupled to the light source 5 and which analysesthe polarisation direction of the light flux reflected by the guidedprojectile 3 in order thereby to calculate at any instant its rollangle.

The laser constituting the light source 5 is pointed at the guidedprojectile 3 by a conventional trajectography system. Moreover, itoperates in pulse mode in order to transmit guidance commands to theguided projectile 3.

The guided projectile 3 is ejected from the gun 2 with a roll rotationmovement. It comprises, as represented in FIG. 2, at least one lateralimpeller consisting of a lateral gas-ejection orifice 10 which may beplaced at the center of gravity and which is coupled to a gas generatorloaded on board the projectile by the agency of a valve opened by pulsesunder the control of a guidance device responding to the commandstransmitted by the pulses from the laser. On instruction, the valveallows a blast of propelling gas to escape during a very short period oftime via the lateral ejection orifice. This moves the projectile 3laterally, which is caused to deviate towards the direction in which thethrust from the gas blast has taken place and departs from its formertrajectory 11 in order to adopt a new trajectory 12. Of course, in orderto be able to use the lateral impeller or impellers judiciously, it isnecessary to know the roll angle of the projectile 3 at any instant.

In order to achieve this, the guided projectile 3 comprises, on its rearface, as represented in FIG. 3, a catadioptric system 15 equipped with apolarizer 16.

This catadioptric system 15 may be, as represented, a reflectortrihedron produced with the aid of a cube corner. This is an opticalinvariant which reflects the light beams received back in theirdirections of incidence. The polarizer 16 only allows the component of alight beam to pass which has a linear polarization parallel to its ownpolarization direction which is tied to the attitude of the projectile.

With a light source 5 generating an incident beam having a verticallinear polarization, the polarizer 16 only allows the beam to passcompletely when its polarization direction is vertical, which occurstwice per turn of the roll of the projectile 3. At this moment, thecatadioptric system 15 receives the beam from the light source 5 withoutit being attenuated by the polarizer 16 and reflects it back in itsdirection of incidence to the light-flux analyser 6 independently of theattitude of the projectile according to the other pitch and yaw angles.As the projectile rotates about itself, the polarizer 16 causes theintensity of the flux of the light source 5 reflected back to thelight-flux analyzer 6 to vary, as represented in FIG. 4, according to asine-squared law of frequency twice the rotation frequency of theprojectile about the roll axis. This sine-squared wave is phase-lockedto within, with maxima corresponding to the polarization direction ofthe polarizer 16 passing through the vertical. By graduating from 0 to 2the curve of intensity variation of the light flux reflected by theprojectile 3, as has been done in FIG. 4, it is therefore possible toknow at any instant the roll angle of the projectile 3. This is what iscarried out by the light-flux analyzer 6 by means of conventionalfiltering techniques. The uncertainty of in the phase locking may beremoved by a trial of one impeller and by detecting, by trajectography,the direction of the deviation caused to the trajectory of theprojectile.

The light source 5 may also generate a circularly-polarized beam. Thelight-beam analyzer 6 then comprises a polarizer disposed aspolarization analyzer in front of an optical receiver sensitive to theintensity of the light flux received. As in the previous case, theintensity of the light flux detected by the optical receiver variesaccording to a sine-squared wave of frequency twice the rotationfrequency of the projectile about the roll axis.

The measurement of the roll angle of the projectile can be disturbed byinopportune variations in the intensity of the light flux reflected bythe projectile because of the presence of natural turbulence orturbulence caused by the wake of the projectile. However, it is possibleto compensate for this, as is shown by the optical device for measuringthe roll angle in FIG. 5.

This FIG. 5 shows a light source 5 which illuminates a guided projectile3 and which is located at a firing station, in the vicinity of alight-flux analyzer 6 analyzing the flux of the light source 5 reflectedby the projectile 3 in order to calculate therefrom the roll angle ofthe projectile 3 at any instant.

The light source 5 is a laser which generates a circularly-polarizedlight beam.

The projectile 3 carries, on its rear face, a catadioptric systemequipped with a polarizer, which are not visible in FIG. 5, such that itreflects back to the light-flux analyzer 6 a linearly-polarized lightflux whose polarization direction depends on its roll angle.

The light-flux analyzer 6 comprises: a splitter 60 such as asemi-transparent mirror which splits the light flux reflected back bythe projectile 3 into two equal portions, a first optical receiver 61preceded by a polarizer 62 which intercepts that one of the portions ofthe light flux which is supplied by the semi transparent mirror 60, asecond optical receiver 63 which directly intercepts that other portionof the light flux which is supplied by the semi transparent mirror 60, aturbulence-compensating circuit 64 dividing the signal supplied by thefirst optical receiver 61 by the signal supplied by the second opticalreceiver 63 and a circuit 65 for estimating roll angle employing thesignal supplied by the turbulence-compensating circuit 64.

The two optical receivers 61, 63 may also be juxtaposed, which avoidsthe use of the splitter.

Optionally, the splitter 60 and the polarizer 62 may be one and the samedevice.

The polarizer 62 operates as a polarization analyzer. It allows thatportion of the light flux to pass which reaches it coming from theprojectile 3 if the linear polarization of this portion is parallel toits polarization direction, blocks it if the linear polarization of thisportion is perpendicular to its polarization direction and, in the otherintermediate cases, only allows a proportion to pass whose magnitudedepends on the angle between the polarization directions according to anaforementioned sine-squared law.

The first optical receiver 61, which is sensitive to the intensity ofthe light flux received by the agency of the polarizer 62, supplies anoutput signal S1 whose amplitude represented in FIG. 6 varies as asine-squared wave of frequency twice the rotation frequency of theprojectile about the roll axis, which curve is modified by a significantparasitic modulation due to fluctuations of light intensity of the beamreflected by the projectile because of natural turbulence and turbulencecaused by the wake of the projectile.

The second optical receiver 63, which is directly sensitive to theintensity of the light beam reflected by the projectile, supplies anoutput signal S2 whose amplitude represented in FIG. 7 is insensitive tochange in the polarization direction of the beam and depends solely onintensity fluctuations of the beam due to turbulence.

The turbulence-compensating circuit 64, by dividing the signal S1 whichis amplitude modulated by the rotation about the roll axis of theprojectile and by the turbulence, by the signal S2 which is modulatedonly by the turbulence, enables a signal S3 to be obtained whoseamplitude represented in FIG. 8 varies only as a function of the rollangle of the projectile, as a sine-squared wave of frequency twice therotation frequency of the projectile.

The circuit 65 for estimating the roll angle is a conventional filteringdevice circuit which employs, for example, a predictive filter centeredon twice the rotation frequency of roll of the projectile, which isknown at the start when the projectile leaves the gun.

FIG. 9 illustrates a complementary optical device which enables, whenthe light source 5 and the light-flux analyzer 6 are laterally offset inrelation to the firing axis of the projectile 3, the ambiguity of to beremoved from the measurement of the roll angle of the projectile 3 fromthe light flux reflected by the catadioptric system equipped with thepolarizer without having to carry out the trial of an impeller.

This complementary device consists of a reflector dihedron 20, forexample a 90° prism, positioned on the side of the projectile 3 andturned towards the rear of the latter, with its edge normal to the axisof the roll-type rotation of the projectile 3. The reflector dihedron 20reflects according to the axis of incidence only the light rays arrivingperpendicularly to its edge. The light rays emitted in its direction bythe light source 5, which is laterally offset in relation to the firingaxis of the projectile 3, are only perpendicular to its edge twiceduring each turn of the roll of the projectile 3. As on one of theseoccasions it is hidden from the light source 5 and from the light-fluxanalyzer 6 by the body of the projectile 3, it reflects the beam fromthe light source 5 back to the light-beam analyzer 6 only once per turnof the roll of the projectile while the latter has a particular rollangle. The accuracy is tied to the divergence of the light source 5which, in any case, has to be small in order to have a long range.

By virtue of the reflector dihedron 20, the light-beam analyzer 6receives, in addition to the light flux reflected by the catadioptricsystem equipped with the polarizer, a light pulse which appears on eachturn of the roll of the projectile for a particular value of the rollangle and which enables the ambiguity of π to be removed from themeasurement.

From the moment of its departure, the projectile is illuminated by thebeam from the light source. The light-flux analyzer receives a lightflux reflected back by the catadioptric system equipped with a polarizerand light pulses reflected back by the reflector dihedron which enablethe roll angle to be measured without ambiguity. When the projectilegets further away, the light pulses reduce in intensity and the phase ismaintained by predictive filtering centred on twice the rotationfrequency.

The optical roll-angle measurement device which has just been describedhas the advantage of transferring all the complexity, that is to say theprocessing circuits, to the firing station and not onto the expendableprojectile. The components on the projectile are simple and robust andthey withstand the high acceleration.

We claim:
 1. An optical system for measuring the roll angle of aprojectile, comprising:means for launching the projectile; a lightsource located at the launching means for projecting a light beam towarda launched projectile, along an axis laterally offset from an axis ofthe beam, and illuminating the rear of the projectile; catadioptricmeans mounted to the rear of the projectile for reflecting light backfrom the source; a polarizer located at the rear of the projectile,intermediate the catadioptric means and the launching means, forpolarizing light reflected back toward the launching means; a dihedronreflector positioned on a radially outward section of the projectile,and having an edge normal to the roll angle of the projectile, forproducing a pulse of light reflected back to the source, once for eachcomplete roll of the projectile, when the source and dihedron reflectorare unobstructed by the projectile body; and light flux analyzer meanslocated at the launching means for calculating projectile roll anglefrom the polarization direction of the light flux reflected back fromboth the catadioptric means and the dihedron reflector, at the rear ofthe projectile.
 2. The system set forth in claim 1 wherein the fluxanalyzer means comprises:first optical means for measuring the lightflux directly reflected from the rear of the projectile; second opticalmeans for measuring the polarized light flux reflected from the rear ofthe projectile, via the polarizer; first circuit means compensating forthe fluctuations of light flux due to turbulence, and splitting a signalsupplied by the second optical means; and second circuit means,utilizing a signal from the first circuit means for calculating the rollangle.